Above is the spectrum of the 540 Hz input signal. The signal frequency was adjusted to give the narrowest spectral peak at the 30 kHz sample rate, allowing the ADC noise and distortion to be seen without being masked by spectral leakage "skirts" from the signal component, and without the need for windowing. (All visible noise and distortion arise in the ADC, since the signal source itself is known to be substantially better.)

TECHNICAL:
The "board" consists of a passive network of just 17 resistors wired into a DB25M connector shell. The network forms an 8-bit R-2R resistor ladder DAC, which the LPTX driver controls as a Successive Approximation Register (hence the SAR model name) to perform the ADC function. Basically, the output of the DAC is summed with the input signal at a digital input port. The DAC bits are adjusted for each sample so that its output plus the signal just balance to keep the port at its switching threshold. From the required DAC setting at threshold the corresponding input level is then determined, which is reported to Daqarta as the ADC input.

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